The manufacture of steel using an electric arc furnace is a highly advantageous process in the modern steel industry. A drawback in the EAF manufacturing of steel is the production of EAF dust waste by-product, which is an EPA listed hazardous waste (K061). During melting and refining of steel in electric arc furnaces, large amounts of galvanized scrap material can be fed into the EAF process. Inherent in the process for making steel by the use of electric steel furnaces with submerged arcs is the liberation of zinc, iron, and other metal values as EAF dust in the off-gas leaving the furnace. To protect the atmosphere, these particles are removed in baghouses, cyclones, scrubbers, and other similar devices. Due to the high proportion of zinc in the dust, it is especially desirable as a source of zinc values for sale. The dust consists of fine particles of iron oxides, ferrites, calcium oxide, and silica, metal chloride and oxide particles of nonferrous metals such as zinc, lead, cadmium, and silver, which vaporize at the high temperatures of the molten steel bath, and which are recovered in the dust.
The feeding of fine particles, containing unwanted metals, back to the arc furnace is not economically viable. As the amount of recycled dust increases, the energy requirement for reducing and melting the iron and other metals from the dust increases, causing melt chemistry problems and decreasing furnace refractory life. Traditionally, this dust has been considered a waste material and has been disposed of in landfills. Recovery techniques used previously generate waste by-products or products not suitable for their intended claimed use and do not offer sufficiently high recovery yield of the metals and products in the EAF dust.
The rapid growth of the EAF steel process has made EAF dust one of the fastest growing and largest environmental problems worldwide. The landfill disposal method is becoming more expensive because of increasingly stringent Environmental Protection Agency (EPA) regulations. The chemical nature of these dust particles are such that they classify as listed hazardous waste, based on the toxicity test prescribed by the United States Environmental Protection Agency. The toxicity concern is related to the presence of lead, cadmium, and chromium.
At present, there are approximately 925,000 tons of this hazardous waste generated annually in the United States and an additional 3,000,000 tons generated annually in the rest of the world. EAF produced steel comprises forty-five percent (45%) of the total U.S. steel production. It is expected to become the major source of steel produced in the U.S. within the next few decades. At present, approximately one-half of the U.S. production of EAF dust is being land filled.
It is a primary purpose of this invention to recover metal values from this steel-making flue dust, and particularly to recover zinc oxide suitable for rubber compounding, and additionally to provide a means for the separation and recovery of other materials in the dust, with minimal environmental impact.
There is also a similar but lower concentration zinc contaminated dust which is derived from the other major process for steel manufacturing, the basic oxygen furnace or basic oxygen process (hereinafter BOF). Because the levels of toxic metals such as cadmium, lead and zinc are lower than current toxicity cutoff levels, BOF dust is not currently classified by the EPA as hazardous. However, BOF dust may be classified as hazardous in the future and its non-iron contaminants, like zinc, make it difficult to utilize in current steel manufacturing, resulting in substantial worldwide stock piles of BOF dust.
With the resource of EAF and BOF dust readily available, and based on environmental need, this process was developed to economically recover nonferrous metals such as zinc, lead, and cadmium from these steel plant dusts. The iron oxide, depleted of these metals, can be recycled back to the steel furnace. Since the tonnage of this raw material is substantial, it represents an important source of zinc, lead and iron metals.
There are also waste dusts and metal sludges available from zinc and copper recovery and extractions processes and other metal processes which also represent valuable sources of non-ferrous metals.
Accordingly, there is a need to develop a hydrometallurgical chloride based process to economically recover valuable metals from metal bearing waste and ores which will solve the problem of unwanted ion contamination of the pregnant leach solution, thus insuring that finished products have extremely low levels of contamination of unwanted ions. Chloride leach processes offer apparent advantages over sulfur based chemistry, such as avoiding roasting, sulfuric acid regeneration, and unwanted waste by-products. Even though these processes offer the opportunity to be environmentally friendly, they have not had much economic success. The need to find a chloride based chemistry process is especially crucial for metal bearing feed stocks that contain chloride. Previously developed hydrometallurgical chloride based leaching processes have required the addition of costly metal chloride salt additives (components like ferric chloride or cupric chloride) or hydrochloric acid, which cause unwanted ions in the pregnant leach solution that have to be removed prior to subsequent metal extraction recovery steps. Other previously developed chloride processes contain ammonium, with resulting safety concerns and the potential for unwanted ammonium compounds. These currently available processes utilize high temperatures, high pressures, and/or highly acidic conditions.
The prior art, as described in U.S. Pat. No. 1,863,700, teaches that zinc oxide/oxychlorides produced from simple precipitation with calcium hydroxide contain as little as 1% chlorides and 3-5% calcium. The patent further teaches a process where a zinc product can be produced with 1-2% calcium and 0.8% chlorides. Such a product thus contains 7% Simonkolleite or other oxychlorides based on the chloride stoichiometry and thus contains less than 92% pure zinc oxide, which is unacceptable for rubber grade zinc oxide and other high purity zinc oxides as well as chloride intolerant zinc refining processes. In contrast, the process of the present invention converts Simonkolleite and other zinc oxychlorides to zinc oxide, meeting specifications and suitability for use as rubber grade zinc oxide with more than 99% pure zinc oxide. The zinc oxide has less than 1,800 ppm chlorides and usually less than 1000 ppm and has a particle size of 0.05 micron to 0.5 micron. The process produces an active zinc oxide with a surface area of 10-70 m2/gram, providing better reactivity and economy compared to French processed zinc oxide. (See “Active Zinc Oxide—the Advantage” by Dr. Harry Rothmann and L. Bruggemann-Sprit und Chemische Fabrik, Germany Tire Technol. Int., p. 118 (1997)). This large amount of surface area offers a more reactive product allowing its use at lower levels than is currently practiced, with improved economics and in processes where French Process zinc oxide has not been acceptable and/or viable.
In the present invention, a pyrolysis process is described where in an iron bearing material containing non-ferrous metals (specifically including but not limited to EAF Dust-KO61) is reduced of its oxides and efficiently stripped of its non-ferrous metals in the presence of carbon and calcium; and a hydrometallurgical process is described utilizing an atmospheric calcium chloride leach to selectively recover from various metal feed stocks (consisting of elemental metals, metal oxides, metal ferrites, metal hydroxides, metal carbonates, metal sulfate/sulfur compounds, and their hydrates, specifically including but not limited to EAF Dust-KO61) the following components: zinc, lead, cadmium, silver, copper and other valuable metals to the exclusion of iron, magnesium, halogen salts and other unwanted elements. The pyrolysis process solves the problem of high levels of non-ferrous metals left in the iron rich material produced from KO-61 when not using calcium.